Modified styrenic polymer beads

ABSTRACT

Styrenic polymer beads that are useful in forming foamed cups with improved strength and thermal properties are prepared by forming an aqueous suspension of initial styrenic polymer beads and adding thereto an emulsion of a comonomer solution of styrenic monomer and divinylbenzene, which emulsion also contains free-radical-producing catalysts, and the suspension with added monomers is heated to copolymerize the styrenic monomer and divinylbenzene within and on the initial beads and form modified beads.

This is a division of application Ser. No. 802,901, filed Nov. 29, 1985,now U.S. Pat. No. 4,622,346.

BACKGROUND OF THE INVENTION

The present invention is directed to a process for forming styrenicpolymer beads that form foams having high strength and thermalproperties. Such high strength foams are useful, for example, in theproduction of cups for drinking.

The formation of styrenic polymer beads by an aqueous suspension processis a well known and commercial practice. Such production is generallyeffected by the suspension polymerization of a styrenic monomer in anaqueous medium with an oil-soluble polymerization catalyst, using asuspending system comprising of a finely divided, difficultlywater-soluble inorganic phosphate, such as tricalcium phosphate, inconjunction with an anionic surface-active agent, such as sodiumdodecylbenzene sulfonate, as a modifier. Such suspension polymerizationsystems are described, for example, in Grim Patent, U.S. Pat. No.2,673,194, the contents of said patent being incorporated by referenceherein. Such styrenic polymer beads are made expandable by impregnationwith a suitable blowing agent.

The foamed cup industry is based on expandable polystyrene (EPS) beads.The base polystyrenic polymer beads used for cups are prepared by anaqueous suspension polymerization in order that the polystyrenic polymermay be recovered as beads which can be screened to a relatively precisebead size. Because of the thin walls in cups, relatively small headsizes are required in a strict range, such as through 35 on 70 mesh(U.S. Standard Sieve). The conventional polystyrene used for EPS beadsfor cups are characterized by a weight average molecular weight (Mw) ofabout 250,000-350,000 with a polydispersity (M_(w) /M_(n)) of about 2-3.Such polystyrenes generally have melt flows (M.I., Condition L) in the2.0-6.0 g/10 min. range. The intrinsic properties of such polystyrenesputs a limit on the molding conditions for producing cups as well as onthe cup properties such as strength. Past efforts to utilize highermolecular weight polystyrenes (350,000-500,000) with M.I. of 1-2, withsimilar M_(w) /M_(n) in order to improve on heat sensitivity and cupstrength has generally resulted in poor overall performance in regard tocup processing capabilities as well as cup appearance.

BRIEF SUMMARY OF THE INVENTION

We have now found that improved stiffness, heat tolerance and resistanceto leakage for a given density of cup relative to current commercialpolystyrene cups can be realized by preparing EPS-type beads which havebeen modified with a styrenic monomer-divinylbenzene (SM/DVB) copolymer,wherein said SM/DVB copolymer is more or less uniformly dispersedthroughout the initial styrenic polymer bead or is concentrated near orat the surface of the bead. The concentration gradient of SM/DVBcopolymer in the styrenic bead is controlled by the process conditions.Depending on the formulary and the polymerization conditions, the SM/DVBcopolymer present may be high molecular weight and toluene soluble, ortoluene insoluble gels, or a combination of both.

The improvements in heat tolerance, stiffness and resistance to leakageappear to be related to a variety of modified bead morphologies. Whenthe SM/DVB copolymer is concentrated primarily at or near the beadsurface, the improvements may result from a molecular weight and densitydifferential in the expanded bead, with the molecular weight and hencedensity of the surface being greater than the core.

Alternatively, the improvements result, when the SM/DVB copolymer ismore or less uniformly dispersed, to form (a) varying degrees ofsemi-interpenetrating networks of crosslinked SM/DVB copolymer with theinitial styrenic polymer or with the initial styrenic polymer andsoluble, lightly crosslinked SM/DVB copolymer or (b) lightly crosslinkedessentially soluble SM/DVB copolymer which increases the overallmolecular weight and polydispersity of the final bead.

DETAILED DESCRIPTION OF THE INVENTION

The present process provides a means for forming SM/DVB modifiedstyrenic polymers which can be converted to foams with improvedstrength, thermal properties and resistance to leakage by copolymerizinga SM/DVB solution absorbed near or at the surface or more or lessuniformly dispersed throughout pre-screened styrenic polymer beads.

The process comprises (a) forming an aqueous suspension of pre-screened,initial styrenic polymer beads with the aid of a suitable suspendingagent system; (b) forming a comonomer solution of a styrenic monomer anddivinylbenzene; (c) forming an emulsion comprising a suitable emulsifierand said comonomer solution with free-radical-producing catalystsdissolved therein; (d) adding the emulsion to the styrenic polymer beadsuspension at 25°-75° C. and allowing said beads to absorb thecomonomers and catalysts either near or at the surface of said beads orallow the beads to absorb and equilibrate so that the comonomers andcatalyst solution is more or less uniformly distributed throughout thebeads; (e) heating the suspension of styrenic beads containing theabsorbed comonomers and catalysts to an intermediate polymerizationtemperature to give firm beads; and (f) heating said bead suspension toa temperature of 115° C. to about 135° C. to substantially complete thepolymerization of said styrenic monomer and divinylbenzene.

Step (d) of the process may be varied without detracting from theimprovements obtained in the final product. For example, a portion ofthe emulsified comonomer solution without dissolved catalyst may beadded and allowed to be absorbed into the initial styrenic beads,followed by the addition of the remaining comonomer emulsion containingall of the dissolved catalyst. Conversely, a portion of the emulsifiedcomonomer containing dissolved catalyst may be added initially to thebead suspension, followed by the addition of the remaining emulsifiedcomonomers.

The term "styrenic" as used herein is intended to include styrene,alpha-methylstyrene, nuclear-methylstyrene, para-t-butylstyrene,monochlorostyrene and dichlorostyrene, as well as mixtures thereof, orsuch styrenic polymer beads containing at least 50 per cent of astyrenic moiety and other ethylenically unsaturated monomers, when usedin conjunction with the styrenic polymer beads or styrenic monomer.

The formation of styrenic polmer initial beads is according to knownprocesses and the present invention is directed to a process for formingstyrenic polymer beads by coating on or dispersing throughout suchinitial beads a copolymer of styrenic monomer and divinylbenzene. Theinitial polymer beads must have a weight average molecular weight ofbetween 230,000 and 350,000 with a polydispersity (Mw/Mn) of 2-3.1.

The initial styrenic polymer beads are formed into a suspension in anaqueous medium by dispersing the beads in water, containing suitablesuspending agent system. One such system may be a finely divided,difficultly water-soluble, inorganic phosphate suspending agent such asthose described in Grim U.S. Pat. No. 2,673,194, and include tricalciumphosphate, hydroxyapatite, magnesium phosphate, and the like. Modifiers,such as sodium dodecylbenzene sulfonate can also be added as per theGrim Patent. The amount of such suspension agent added is about 2.5-4.0percent by weight of the final product, with about 3 percent by weightpreferred. Another such system is a polyvinyl alcohol suspending agent,such as is sold under the trademark "Vinol 540", in an amount of about0.07-0.30 percent by weight, with a preferred amount of about 0.10percent, based on the final product weight. Other organic suspendingagent systems include hydroxyethyl cellulose and polyvinyl pyrrolidone.Equally useful systems employ combinations of inorganic and organicsuspending agents, for example tricalcium phosphate and polyvinylalcohol. Mixture of these systems can also be used. The initial styrenicpolymer beads may also have additional additives, such as internalfast-cool agents, pigments and dyes, stabilizers, anti-lump agents,self-extinguishing agents, plasticizers, and polymeric additives, suchas minor amounts (0.05-0.50 wt. %) of polyethylene, polypropylene, andin particular, polywaxes such as low molecular weight (Mn 650-1500),narrow MWD, crystalline (densities 0.93-0.96) polyethylenes and similarmolecular weight Fisher-Tropsch waxes.

There is also prepared a solution of styrenic monomer anddivinylbenzene. The divinylbenzene comonomer may be any of the threeisomers thereof, or preferably, may be any of the commercially availablemixtures thereof.

The amount of styrenic monomer and divinylbenzene present in thesolution will depend upon the amount of the copolymer to be dispersed inand on about the initial styrenic beads but should be such that thefinal beads produced comprise 65 to 92 percent by weight of the initialstyrenic polymer beads and 8 to 35 percent by weight of the copolymer.The copolymer itself should be formed from a mixture of styrenic monomerand divinylbenzene such that the copolymer is formed from 99.15 to 99.98by weight styrenic monomer and 0.02 to 0.85 percent by weight ofdivinylbenzene. This solution containing dissolved catalysts is thenconverted to an emulsion with the addition thereto of a suitableemulsifier. Suitable emulsifiers may be the sodium alkylbenzenesulfonates, such as dodecylbenzene sulfonate, as well as the variousnonionic surfactants comprising polyoxyethylene mono-ethers andmono-esters, such as polyoxyethylene sorbitan monolaurate, wherein thepolyoxyethylene portion may have from 20 to 50 moles of ethylene oxide.Examples of the mono-ethers are the ethylene oxide condensates of octylor nonylphenol. The catalysts are a mixture of at least two free-radicaltype materials comprising a primary low-temperature initiator having a10-hour half-life temperature of between 60° and 80° C. and a secondaryhigh-temperature initiator having a 10-hour half-life temperature ofbetween 95° and 110° C. Thus, a mixture of benzoyl peroxide and t-butylperbenzoate which have 10-hour half-life temperature of 73° and 105° C.,respectively, can be used. Azo catalysts having suitable 10-hourhalf-life temperatures are also useful.

The aqueous comonomer catalyst emulsion, so produced, is addedcontinuously to the aqueous suspension of styrenic polymer beads over aperiod of about 0.30-2.5 hour, while heating the suspension withstirring, at a temperature of between 25° and 75° C. and maintainingthat temperature during the addition. The suspension of styrenic beadscontaining absorbed comonomer/catalyst may then be either heated to anintermediate temperature of about 80°-95° C. over a period of 10-45minutes directly after the addition of comonomer/catalyst emulsion iscomplete or it may be allowed to stand at 25°-75° C. for various timeperiods to permit the absorbed comonomer/catalyst to diffuse more orless uniformly throughout the beads before heating to the intermediatetemperature of about 80°-95° C. Then the suspension is heated further atthe intermediate temperature of about 80°-95° C. until the beads arefirm or for a period of 0.10-6.0 hours. The morphology of the resultingSM/DVB modified beads can be varied during this comomoner addition stageand polymerization to firm beads and will be dependent on a number offactors such as the rate of comonomer addition, temperature at the timeof addition, and the time interval before the temperature is increasedto produce a significant copolymerization rate. The morphology thereforemay be varied wherein the SM/DVB copolymer (either toluene soluble orinsoluble or mixture thereof) may be more or less concentrated towardthe surface, as a consequence of fairly rapid addition and rapidattainment of significant copolymerization rate to obtain firm beads, orthe SM/DVB copolymer may be more or less uniformly dispersed throughoutthe bead, as a consequence of the length of time allowed for absorptionand equilibration of the comonomers in the initial styrenic bead tooccur before permitting significant copolymerization to form firm beads.

After heating at the intermediate temperature of about 80°-95° C. hasbeen completed, the suspension is then further heated, such as for fourhours at 115° C. or one to three hours at 135° C., to reduce the amountof unpolymerized monomers to less than 0.25%, preferably less than0.05%.

Alternatively, the comonomer solution can be divided into two portions,with the catalysts added to one portion only. For example, the portionwith no catalyst may consist of approximately 2/3 of the total solutionand, after emulsification, is added at any desired temperature between25° and 75° C. to the suspended polystyrenic polymer beads. Theremaining 1/3 of the solution containing all of the catalysts isemulsified, and added to the suspension at the same temperature or adifferent temperature. A further alternative could be the addition ofthe catalysts-containing portion first, followed by the addition of theportion not containing catalysts.

Again, the rate of addition may be rapid or slow, and the absorptionequilibration times may be varied in order to control the morphology inthe final bead product. The comonomers absorbed on or in the styrenicbeads are then polymerized at the intermediate temperatures of about80°-95° C. and finished off at 135° C. as before.

The amount of toluene insoluble gels and degree of crosslinking in thegels will be primarily a function of the amount of comonomer to beadded, the DVB concentration and polymerization conditions. Relativelyhigh DVB levels will generally cause essentially all of the comonomersto be converted to gels along with some gel formed via grafting with theseed styrenic polymer. It is recognized by those skilled in the arthowever that gelation primarily of the added styrene-DVB comonomers canbe delayed by reducing the amount of DVB or the degree of polymerizationof the added monomers by using chain transfer agents or increasedpolymerization rates. The extent of crosslinking is also controlled bythese variables. Thus extensive crosslinking with the formation of"tight" networks in the gel can be obtained by using relatively highlevels of DVB and avoiding the use of chain transfer agents. Decreasingthe DVB concentration of course will reduce the extent of crosslinkingand eventually the amount of gels formed with a given DVB comonomermodifying solution. The modifying DVB copolymer can comprise from 8-35%by weight of the final bead product and may be essentially toluenesoluble, high molecular weight (Mw>350,000) or may be in the form oftoluene insoluble gels or a mixture of toluene insoluble gels andtoluene soluble DVB copolymer.

Consequently, for a given percentage of DVB copolymer desired in thefinal beads (ranging from 8-35 percent by weight), the DVB/catalystsconcentration and polymerization conditions can be varied to produceeither high, intermediate or very little or no levels of gels. However,the DVB concentration lends itself to producing major changes readily.For example, in the absence of chain transfer agents and relativelymoderate polymerization conditions, the use of about 0.35 percent DVB orhigher based on styrenic monomer will convert essentially all of theSM/DVB comonomer to gels with the higher DVB levels causing the gelnetwork to become "tighter". Reducing the DVB levels will cause adecrease in gels and a gradual increase in the molecular weight andpolydispersity of the toluene soluble components until at very lowlevels of about 0.02% DVB (based on styrene monomer) little or no gelsare formed.

The best overall balance of cup properties, such as molding cycle, rimstrength improvement, heat tolerance, appearance and resistance toleakage is obtained when the percentage by weight of gels in the overallfinal bead is in the range of approximately 8-27%. When the gel level isat the high end of the range, then the gel network should be relatively"loose" in order that expandability and molding characteristics are notimpaired. When little or no gel (<2.0%) is formed, then the molecularweight of the final bead should be high, >350,000, and thepolydispersity broad, >2.8.

In general, the higher the weight percent of SM/DVB copolymer modifierin the final bead, the lower the DVB concentration needed to obtainfinal beads with the desired range of characteristics which result incups with improved strength, heat tolerance and resistance to leakage.

Beads useful for cups because of the thin walls are of necessity muchsmaller than those generally found useful for other EPS applicationssuch as packaging, insulation boards and the like, which generallyrequire lower densities and thicker parts. As a consequence, thedesirable bead size for cups will fall in the mesh range of through 30on 80, with generally only a relatively low percentage being on 35 mesh.Consequently, since the process is a bead growth process, pre-screenedbeads are generally utilized with the average seed bead size beingselected on the basis of the amount of DVB copolymer modificationsdesired in the final product. The greater the percentage modificationwith DVB copolymer the smaller the required seed bead needed to producea final bead product in the desired average bead size useful for cups.This poses no serious problem, since such seed beads are available viacurrent commercial processes.

The modified beads as formed may be separated from the aqueoussuspension, washed, dried, and screened, if necessary. Thecopolymerization process is carried out so that essentially no secondarybead formation (from the SM/DVB comonomer solution) occurs.Consequently, by the judicious selection of pre-screened beads, and theamount of the comonomers added, the bead growth can be controlled togive a final bead size suitable for a given application, such as cups.The modified beads may be impregnated directly in the same reactorwithout further bead screening. This has significant economic advantage.

The beads are made expandable by the usual impregnation techniques usingblowing agents which are gases or produce gases on heating, such asaliphatic hydrocarbons containing from 4 to 6 carbon atoms in themolecule, such as butane, pentane, cyclopentane, hexane, cyclohexane andthe halogenated hydrocarbons which boil at a temperature below thesoftening point of the polymer. Mixtures of these agents may also beused. The blowing agents are incorporated by conventional processes,such as that described in U.S. Pat. No. 2,983,692.

The present invention is further illustrated in the following examples,wherein parts and percentages are by weight unless otherwise indicated.

EXAMPLE I A. Formation of 21.4% Copolymer in and on Polystyrene Beads

A series of examples was carried out as follows. An initial polystyrenebead slurry was prepared in a 2-liter resin kettle having an agitator,reflux condenser, combination baffle/thermometer wells and a monomerfeed port, by adding thereto 600 g water, 16.0 g polyvinyl alcoholsolution (5% active), 12 g tricalcium phosphate and 440 g polystyrenebeads of through 35 mesh and on 50 mesh bead size (U.S. Standard Sieve).The weight average molecular weight (Mw) of the polystyrene was 270,000.The slurry was heated, with stirring at 300 rpm, to 70° C. A comonomersolution (I) was formed by mixing 120 g styrene, and 0.821 gdivinylbenzene, (80.6% active), and 0.074 g of Perox Violet dye. Anemulsion (II) of (I) was prepared from 80 g (I), 120 g water, and 0.8 gof polyoxyethylene (20) sorbitan monolaurate and mixed for 5 minutesusing a Polytron high intensity stirrer. The emulsion (II) was addeddropwise over a period of about 7 minutes to the suspension at 70° C. Anemulsion (III) of a solution of catalyst was then added. This emulsion(III) was formed by dissolving 0.207 g benzoyl peroxide and 0.063 g oft-butyl perbenzoate in 40.8 g comonomer solution (I) with mild stirring.The resultant solution then had added thereto 50 g water containing 0.5g polyoxyethylene (20) sorbitan monolaurate. This mixture was intenselymixed with the Polytron homogenizer for several minutes to form theemulsion (III). The emulsion (III) was added continuously to the slurryover 33 minutes at 70° C. Then the mixture was heated to 90° C. overapproximately 15 minutes and maintained at 90° C. for 4 hours. A sampleof beads removed after 15 minutes at 90° C. were firm, uniform in color,with no evidence of secondary bead formation.

The suspension was cooled to 35° C. and transferred in approximately 200g. portions to 12-ounce bottles containing 1 g. tricalcium phosphate and1 ml of 1% sodium dodecylbenzene sulfonate. The bottles were purged withnitrogen, sealed with crown caps and rotated end-over-end in an oil bathheated to 135° C. over 1.5 hours and at 135° C. for 1.5 hours. Aftercooling to room temperature, the bottles were opened and the contentsacidified to pH 1.0 with hydrochloric acid. The beads were separated andwater-washed on a 100 mesh sieve. The beads were spherical,non-agglomerated, uniform in color, and were within the proper sizerange, i.e. >99.5% passing through a 35-mesh sieve and retained on a50-mesh sieve, desired for molding of foam cups from expandable beads.

B. Impregnation of Modified Beads

To render the beads, from the series of examples, expandable, aliquotsthereof were impregnated by n-pentane in 12 oz. bottles heated at 105°C. for two hours while being rotated end-over-end in an oil bathaccording to the following formulations:

100 g of beads of polystyrene having 21.4% total weight ofstyrene/divinylbenzene copolymer from above.

100 g water

2.0 g tricalcium phosphate

0.02 g sodium dodecylbenzene sulfonate

0.15 g polyoxethylene(20)sorbitan monolaurate

7.8 g n-pentane

The bottles were cooled to room temperature, opened, acidified withhydrochloric acid, centrifuged, water-rinsed, tray-dried untilfree-flowing. Volatile content of the impregnated beads was 6.14% withonly 0.04% of water. The beads were then treated with 300 ppm ofsilicone oil and 1000 ppm of zinc stearate in a twin shell blender for30 minutes and then batch expanded in a 5 gallon Buccaneer pre-expanderto the pre-expanded beads of density 4.24 pcf.

C. Formation of 12 and 30% Copolymer in and on Polystyrene Beads

In a manner similar to A above, polystyrene beads containingapproximately 12% and 30% styrene-divinylbenzene copolymer based onfinal bead weight were prepared, impregnated and expanded. At all threecopolymer modification levels (12, 21.4, and 30%) the tert-butylperbenzoate concentration (based on styrene monomer) was keptapproximately constant at 0.053%, but the benzoyl peroxide and activedivinylbenzene concentrations were varied in order to assess theireffect on cup properties. The seed beads used in the 30% S/DVBmodification passed through 45 mesh and retained on 80 mesh. Aftermodification, 99.7% passed through 40 mesh.

D. Cup Molding and Evaluation

All of the above-laboratory prepared materials were molded on a cupmachine made by Master Machine and Tool Company using an 8F mold toproduce 8 oz. smooth wall cups at a steam header pressure of 52 psi anda back pressure of 32 psi. The total cycle consists of four stages: (1)fill time--filling the mold, (2) dwell time--time steam heats the moldwall by conduction, (3) cook time--time steam actually passes throughexpanded pre-puff in the mold, and (4) cool time--time it takes forcooling water to cool the mold for cup ejection.

The results of this example are tabulated in Table I. The polystyrenecontrols were molded at a cook time of about 1.5 second, but the beadsmodified with styrene-divinylbenzene copolymer were given a cook time of5.0 seconds. The controls could not withstand the longer cook times.

                                      TABLE I                                     __________________________________________________________________________                        Prepuff             % Improvement over                                        Density                                                                            Cup Weight                                                                           Rim Deflection                                                                        Polystyrene Controls                  No.                                                                              % Copolymer                                                                           % BPO/% DVB                                                                            (Pcf)                                                                              (g.)   (mils)  Cup Wt.                                                                            Deflection                       __________________________________________________________________________     1 12.0    0.173/0.275                                                                            3.46 2.09   398     2.3  3.4                               2 12.0    0.173/0.54                                                                             3.87 2.35   325     4.1  5.5                               3 12.0    0.177/0.825                                                                            3.51 2.08   363     8.3  13.2                              4 12.0     0.20/0.825                                                                            3.80 2.23   335     6.3  10.2                              5 12.0    0.155/0.275                                                                            3.24 1.94   410     8.1  10.9                              6 21.4    0.173/0.276                                                                            3.71 2.27   300     11.0 17.8                              7 21.4    0.173/0.55                                                                             4.19 2.55   235     12.7 21.7                              8 21.4    0.173/0.55                                                                             4.24 2.63   225     11.1 20.5                              9 21.4    0.20/0.55                                                                              4.42 2.75   210     11.0 19.8                             10 21.4    0.173/0.55                                                                             3.98 2.37   259     14.4 23.4                             11 21.4    0.20/0.55                                                                              3.72 2.24   284     14.8 23.5                             12 21.4    0.173/0.55                                                                             3.90 2.35   255     15.8 25.7                             13 30.0    0.20/0.55                                                                              4.88 3.07   168     13.2 20.4                             14 30.0    0.177/0.55                                                                             4.88 3.08   169     12.7 19.1                             15 30.0    0.155/0.275                                                                            3.46 2.28   334     7.2  4.2                              16 30.0     0.20/0.275                                                                            3.39 2.21   340     6.8  10.5                             17 30.0    0.155/0.825                                                                            5.28 3.56   158     4.0  5.9                              18 30.0     0.20/0.825                                                                            4.54 3.00   200     5.4  9.9                              19 30.0    0.155/0.825                                                                            4.53 2.92   224     2.0  1.3                              20 30.0     0.20/0.825                                                                            4.21 2.74   234     6.4  10.8                             __________________________________________________________________________

As can be seen from Table I, in all cases where polystyrene beads weremodified with S/DVB copolymer, the cups had improved cup weight anddeflection compared to the cups molded from the polystyrene controls.The rim deflection of the cups in mils was measured on a Chatillion LTCMTension and Compression Tester, using a deflection speed of 0.35inch/minute at a force of 200 grams.

EXAMPLE II

To illustrate that, since bead size control is so good with thecopolymer modification techniques there is no need to screen the beadsprior to impregnation, a series of runs was made using a single reactorin which both the modification and impregnation steps were carried out.Two methods of modification were carried out as follows:

Procedure A: One step addition of emulsified comonomer-initiatorsolution.

To a 25 gallon stirred reactor was added 69.6 parts of distilled water,78.7 parts of polystyrene beads (Mw 280,000; bead size through 35 and on50 mesh, U.S. Standard Sieve), 6.0 parts of tricalcium phosphate and0.0031 parts of sodium dodecylbenzenesulfonate. The reactor wasmaintained at 25° C., while a homogenized solution of comononers andinitiators was added over 30 minutes. The emulsified (homogenized)solution contained 21.25 parts of styrene, 0.106 parts (80.6% active)(0.40% based on styrene) divinylbenzene, 30.42 parts water, 0.0429 parts(0.20% based on styrene) of benzoyl peroxide, 0.0112 parts (0.053% basedon styrene) of tert-butyl perbenzoate and 0.0192 parts of sodium dodecylbenzenesulfonate. The agitated bead slurry was heated to 90° C. over 30minutes, held at 90° C. for 2.5 hours and then heated to and at 135° C.over 3.0 hours. The beads then contained 21.4% by weight of styrenedivinylbenzene copolymer.

The modified bead slurry was cooled from 135° C. to 105° C., then 0.15parts of polyoxyethylene(20)sorbitan monolaurate in 3.26 parts of waterwas bombed into the reactor, followed by the addition of 7.8 parts ofn-pentane over 1.5 hours. The slurry was held at 105° C. for anadditional 0.5 hours, then cooled to 35° C.; acidified to pH 1 and thebeads were filtered, washed and dried.

Procedure B: Two step addition of emulsified comonomer and emulsifiedcomonomer/catalyst solution.

This procedure was similar to Procedure A except that the modificationsolution was added in 2 steps. Approximately 2/3 of thestyrene-divinylbenzene solution was homogenized without any initiatorspresent, with 21.48 parts of water containing 0.0136 parts of theemulsifier, sodium dodecylbenzene sulfonate, and added to the slurry at70° C. over 10 minutes. The remaining 1/3 of the comonomer solution,containing the initiators was homogenized with 8.94 parts of watercontaining 0.0057 parts emulsifier, and then added over 45 minutes at70° C., and the resulting bead slurry heated to 90° C. and held at 90°C. for 1.5 or 4.0 hours before finishing off at 135° C. The beads wereimpregnated with pentane as in Procedure A.

In both Procedures, A and B, the emulsified monomers can be added eitherat 25° C., or at 70° C., as described, or any temperature in between.Similarly, the times of polymerization at 90° C. could be varied. It wasfound that after the addition of the emulsified comonomer solution,samples of beads removed after 15 minutes at 90° C. were quite firm anduniform in size distribution. Reduction of the polymerization time at90° C. to as low as 1.5 hours was accomplished without any apparentadverse effects on expandability or the cup properties produced.

                                      TABLE II                                    __________________________________________________________________________                Procedure A                                                                              Prepuff                                                                            Cup Rim   % Improvement                           Run         Temp./Time                                                                           Hours                                                                             Density                                                                            Weight                                                                            Deflection                                                                          over PS Controls                        No.                                                                              % BPO/% DVB                                                                            (°C./min)                                                                     90° C.                                                                     (PCF)                                                                              (g.)                                                                              (mils)                                                                              Cup Wt.                                                                            Deflection                         __________________________________________________________________________    1  0.173/0.40                                                                             70/45  4.0 3.45 2.02                                                                              330   16.2 24.5                                                      3.86 2.25                                                                              287   14.1 22.6                                                      5.18 3.06                                                                              174   10.8 18.3                               2  0.20/0.40                                                                              70/45  4.0 3.0  1.79                                                                              395   17.1 23.3                                                      3.84 2.26                                                                              295   12.4 19.6                                                      5.08 3.00                                                                              201   5.1   9.0                               3  0.20/0.40                                                                              70/45  4.0 3.30 1.94                                                                              368   15.3 22.3                                                      4.05 2.36                                                                              268   13.2 21.2                                                      5.25 3.19                                                                              178   6.2  10.1                               4  0.20/0.40                                                                               70/135                                                                              4.0 3.40 2.00                                                                              359   12.7 18.6                                                      4.17 2.41                                                                              278   9.4  15.5                                                      4.82 2.81                                                                              223   6.0  11.2                               5  0.20/0.40                                                                               70/135                                                                              4.0 3.23 1.91                                                                              359   12.2 23.2                                                      3.80 2.22                                                                              301   12.6 20.2                                                      4.97 2.93                                                                              197   8.4  14.7                               6  0.20/0.40                                                                               70/135                                                                              4.0 3.35 2.00                                                                              367   11.5 17.0                                                      3.78 2.21                                                                              310   12.0 18.6                                                      5.25 3.21                                                                              185   3.6   6.1                               7  0.20/0.40                                                                              25/30  2.5 3.33 1.98                                                                              360   13.2 18.9                                                      3.87 2.25                                                                              282   14.4 23.4                                                      5.06 2.96                                                                              173   14.2 24.1                                8*                                                                              0.20/0.30                                                                              25/30  1.5 3.57 2.06                                                                              308   15.2 25.2                                9*                                                                              0.173/0.40                                                                             70/30  1.5 4.14 2.48                                                                              237   9.5  20.5                               __________________________________________________________________________     *3 sec. cook                                                             

Samples modified according to Procedure A were molded into 8 oz. cups asin Example I using 52 psi steam header pressure and 32 psi back pressurewith a cook time of 5 seconds except as indicated in the results inTable II.

Samples modified according to Procedure B were molded into 8 oz. cups asabove using 80 psi steam header pressure and from 34 to 45 psi backpressure with a cook time of 3 seconds, with results shown in Table III.

                                      TABLE III                                   __________________________________________________________________________                Procedure B   90° C.                                                                     Prepuff                                                                            Cup Rim   % Improvement                    Run         Temp./Time                                                                           Temp./Time                                                                           Hold                                                                              Density                                                                            Weight                                                                            Deflection                                                                          over PS Controls                 No.                                                                              % BPO/% DVB                                                                            (°C./min)                                                                     (°C./min)                                                                     (hours)                                                                           (Pcf)                                                                              (g.)                                                                              (mils)                                                                              Cup Wt.                                                                            Deflection                  __________________________________________________________________________    10 0.173/0.40                                                                             70°/10                                                                        70°/45                                                                        4.0 3.76 2.13                                                                              280   16.5 28.2                                                      4.65 2.62                                                                              205   9.0  22.1                                                      3.34 1.91                                                                              318   20.1 31.9                                                      3.34 1.91                                                                              315   20.8 32.6                        11 0.16/0.40                                                                              70°/10                                                                        70°/45                                                                        4.0 3.48 2.03                                                                              320   15.1 24.5                                                      3.70 2.16                                                                              281   15.0 26.4                                                      5.12 3.06                                                                              160   1.6  6.4                         12 0.173/0.40                                                                             25°/10                                                                        25°/45                                                                        1.5 4.02 2.33                                                                              232   15.3 30.5                        13 0.20/0.30                                                                              25°/10                                                                        25°/45                                                                        1.5 3.71 2.13                                                                              290   15.1 25.6                        14 0.173/0.40                                                                             70°/10                                                                        70°/45                                                                        1.5 3.97 2.38                                                                              241   12.5 25.2                        15 0.20/0.30                                                                              70°/10                                                                        70°/45                                                                        1.5 3.83 2.31                                                                              257   12.8 24.4                        16 0.35/0.40                                                                              70°/10                                                                        70°/45                                                                        1.5 3.10 1.81                                                                              415   12.1 17.8                        17 0.35/0.50                                                                              70°/10                                                                        70°/45                                                                        1.5 3.13 1.83                                                                              397   13.3 20.0                         18*                                                                             0.35/0.40                                                                              70°/10                                                                        70°/45                                                                        1.5 3.17 1.86                                                                              444   6.1  8.4                         19 0.35/0.30                                                                              70°/10                                                                        70°/45                                                                        1.5 3.18 1.83                                                                              416   11.2 16.1                        __________________________________________________________________________     *14.1 coating                                                            

EXAMPLE III A. Formation of Polystyrene Beads Modified with 21.4%Styrene-Divinylbenzene Copolymer

To illustrate the effect of divinylbenzene concentration and the benzoylperoxide/divinylbenzene ratio on the amount of gel formation and themolecular weight of the soluble polymer, the series of runs summarizedin Table IV were made.

Runs 7, 9, and 12 were 25-gallon reactor runs. Runs 7 and 12 were madeaccording to Example II, procedure B; Run 9 according to Example II,procedure A. In Runs 9 and 12, however, the suspensions were allowed toequilibrate at 70° C. for one hour prior to heat up to the intermediatetemperature. The remaining runs were conducted in the 2-liter kettle ofExample I.

The BPO/DVB levels were varied as indicated in Table IV, whereas thesecondary initiator, t-BP, was kept in the range of 0.053-0.066% basedon styrene monomer. In all cases, the emulsified comonomer additionswere made at 70° C. but with several variations in the mode, time ofadditions, and hold times at 70° prior to heat up to the intermediatepolymerization temperatures of about 80°-95° C.

Run 1 was conducted according to Example IA except no polyvinyl alcoholwas utilized in conjunction with the tricalcium phosphate suspendingagent and sodium dodecylbenzene sulfonate was used as the emulsifyingagent in place of the polyoxyethylene (20) sorbitan monolaurate.

The reactions conducted in the 2-liter kettle from Example I for Runs2-6, 8, 10 and 11 were as follows:

Prescreened polystyrene beads, 440 g., of through 35 and on 60 mesh,U.S. Standard Sieve, with weight average moelcular weight, M_(w), ofabout 270,000 and M_(w) /M_(n) of about 2.35, were charged to thereactor along with 400 g. distilled water and 12 g. tricalcium phosphateand heated to 70° C. while stirring at 400 rpm. The comonomer/catalystsolution consisting of 120 g. styrene with dissolved divinylbenzene(80.6% active), BPO and t-BP as shown in Table IV was mixed with 160 g.distilled water containing 0.12 g. of dissolved sodium dodecylbenzenesulfonate, and emulsified by intensive mixing for about 1-2 minutes withthe Polytron homogenizer.

In runs 1, 7 and 12, two thirds of the emulsified comonomer solutionwithout catalyst was added at 70° C. to the polystyrene bead suspensionover 20, 10 and 10 minutes, respectively, while maintaining stirring,followed by the remaining one third containing all of the catalystsadded over 30, 45 and 45 minutes, respectively. In the remaining runs,all of the emulsified comonomer/catalysts was added at 70° C. whilestirring at 400 RPM's over approximately 30 minutes. Runs 1, 2, and 7were then heated to the intermediate temperature of 90° C. directlyafter the additions were completed, whereas in all the remaining runs,the suspensions were allowed to soak for one additional hour at 70° C.prior to heat up to the intermediate temperatures. After heat up to theintermediate temperature, generally within 5-15 minutes, and hold timeat such temperature as indicated in the Table, the suspensions were thenheated to and at 135° C. for approximately three hours to complete thecopolymerizations. The beads were recovered as in Example IA, exceptRuns 7, 9, and 12 were impregnated directly in the reactor.

B. Impregnation of Modified Beads

The beads from Runs 1-6, 8, 10, and 11, were impregnated with n-pentaneas in Example IB; the beads in Runs 7, 9, and 12, as already indicated,were impregnated as in Example IIA.

C. Cup Molding and Evaluation

Runs 1-4 and 7 were molded on the Master Cup Machine using an 8 oz. moldas described in Example ID.

Runs 5, 6, and 8-12 were molded on a Thompson Cup Machine using a 6 oz.smooth wall mold at a header pressure of 120 psi and back pressure of 45psi.

The results of the cup evaluations are tabulated in Table V. Thepolystyrene controls were molded with cook times of about 0.9-1.4seconds. Extra cook times of 1.0 additional second resulted in burnedcups. The results in Table V for the S/DVB modified products wereobtained on cups molded with the 1.0 second extra cook time, showingtheir much greater heat tolerance and improved cup strength.

At cup densities of between 3.0 and 4.3 pcf all runs showed improvementsin rim deflection of between 17 and 29%, which translates into cupweight improvements of between 10 and 17% over the polystyrene controls.

D. Characterizaton of Bead Products

The various bead products were separated into toluene soluble andtoluene insoluble components and the soluble component was furthercharacterized as to weight average molecular weight, Mw, andpolydispersity, Mw/M_(n).

Samples of beads, approximately 0.4-1.0 g., were accurately weighed,placed in stoppered flasks along with 200 g. of toluene. These wereallowed to digest, with intermittent swirling, for periods of 3-7 days.The gels were filtered into crucibles onto Celite filter beds andpercent gels quantitatively determined. The soluble component wasrecovered from the toluene filtrate by precipitation into methanol. TheM_(w) and M_(w) /M_(n) were determined on a Waters Gel PermeationChromatograph, Model 150C.

Visually, it was noted that the high DVB (0.55 and 0.40% DVB) beadsswelled less and were generally more easily filtered and retained theirbead identity longer after filtering. The intermediate DVB beads(0.20-0.30%) swelled more and when filtered lost their bead identitymore quickly, and had a greater tendency to clog the filter. The sampleswith 0.10 and 0.04% DVB were essentially soluble in the toluene. Resultsof these runs are shown in Table IV.

                                      TABLE IV                                    __________________________________________________________________________                                     Toluene                                      Run             Coating Procedure                                                                        Hold Temp                                                                           Insoluble                                                                          Soluble                                 No.  % BPO/% DVB (d)                                                                          °C./min                                                                     °C./min                                                                      °C./hrs.                                                                     gels %                                                                             M.sub.w × 10.sup.-3                                                           M.sub.w /M.sub.n                  __________________________________________________________________________    Control                                                                            0.25/--.sup.                                                                             --   --    90°/6                                                                        --   270   2.4                               1    0.30/0.55  70°/20                                                                      70°/30                                                                       90°/4                                                                        24.4 262   2.32                              2    0.175/0.40 --   70°/30                                                                       90°/1.5                                                                      25.4 264   2.49                              3    0.30/0.40  --   70°/30 (c)                                                                   80°/0.5                                                                      22.6 246   2.35                              4    0.30/0.40  --   70°/30 (c)                                                                    90°/0.25                                                                    24.1 259   2.35                              5    0.35/0.40  --   70°/30 (c)                                                                   90°/1.5                                                                      23   258   2.34                              6    0.35/0.30  --   70°/30 (c)                                                                   90°/1.5                                                                      20.3 258   2.54                                7 (a)                                                                            0.25/0.25  70°/10                                                                      70°/45                                                                       90°/1.5                                                                      20.3 270   2.42                              8    0.40/0.20  --   70°/30 (c)                                                                   90°/1.5                                                                      14.3 281   2.65                                9 (a)                                                                            0.40/0.20  --   70°/30 (c)                                                                   90°/1.5                                                                      13.0 295   2.62                              10   0.30/0.10  --   70°/30 (c)                                                                   90°/1.5                                                                      3.3  334   3.0                                 11 (b)                                                                           0.20/0.04  --   70°/30 (c)                                                                   90°/1.5                                                                      1.5  368   2.8                               12   0.25/0.25  70°/10                                                                      70°/45 (c)                                                                   90°/1.5                                     __________________________________________________________________________     (a) prepared in a 25 gal reactor                                              (b) 25% S/DVB comonomers                                                      (c) allowed to soak 1.0 hr at 70° C. after comonomer addition          completed                                                                     (d) active percent based on styrene.                                     

                  TABLE V                                                         ______________________________________                                        Pre-Puff    Cup     Rim       % Improvement                                   Density     Weight  Deflection                                                                              Over PS Controls                                Run No.                                                                              (pcf)    (g)     (mils)  Cup Wt.                                                                              Deflection                             ______________________________________                                        1      3.87     2.35    269     13     22                                     2      4.27     2.59    240     10     17                                     3      3.54     2.11    335     10     15                                     4      3.54     2.08    307     14     24                                     5      3.46     1.87    178     12     23                                     6      3.35     1.81    176     15     29                                     7      3.28     1.99    316     17     28                                     8      3.35     1.86    179     16     29                                     9      3.15     1.78    203     11     20                                     10     3.16     1.74    212     11     21                                     11     3.21     1.80    189     13     24                                     12     3.10     1.71    209     13     25                                     ______________________________________                                    

From Table IV, it can be seen that at the higher DVB levels (0.40-0.55%)the S/DVB comonomers all go into the formation of gels and someadditional gels are formed by some grafting with the initialpolystyrene. The molecular weight and polydispersities of the toluenesoluble polystyrene component is very similar to that of the controlpolymer. At moderate DVB levels (0.25-0.30%), almost all of thecomonomers are converted to gels. At 0.20% DVB, the gels decreasesignificantly and the moleculr weight and polydispersity of the solublecomponent increased moderately. At the lower DVB levels, (0.04-0.10),the gels were quite low, but the Mw and Mw/Mn increased significantly.

What is claimed is
 1. A bead product comprising a polystyrene beadhaving a styrene monomer-divinylbenzene copolymer relatively uniformlydispersed throughout the bead; said copolymer consisting essentially ofstyrene monomer and 0.02 to 0.85 percent by weight of divinylbenzene andsaid copolymer comprising 8 to 35 percent by weight of the total beadproduct; said bead product produced by the process comprising:(a)forming an aqueous suspension of initial styrene polymer beads having aweight average molecular weight of 230,000 to 350,000 with apolydispersity (Mw/Mn) of 2 to 3.1, with the aid of a suitablesuspending agent system; said initial styrene polymer beads selectedfrom the group consisting of polymers of styrene, alpha-methylstyrene,nuclear-methylstyrene, para-tert-butylstyrene, chlorostyrene,dichlorostyrene, mixtures of these, and copolymers of at least 50%styrene and up to 50% of other mono-ethylenically unsaturated monomerscopolymerizable the therewith; (b) forming a comonomer solution of astyrene monomer and divinylbenzene; said styrene monomer being selectedfrom the group consisting of sytrene, alpha-methylstyrene,nuclearmethylstyrene, para-tert-butylstyrene, monochlorostyrene,dichlorostyrene and mixtures thereof; (c) forming an emulsion comprisinga suitable emulsifier and a portion of said comonomer solution; (d)adding the emulsion to the styrene polymer bead suspension at 25°-75° C.to allow the comonomers to be absorbed into the beads; (e) forming asecond emulsion comprising a suitable emulsifier, the remaining portionof said comonomer solution, and free-radical-producing catalysts; (f)adding the second emulsion to the suspension of said beads absorbed withcomonomers at 25°-75° C., at a rate such that the rate of absorption ofcomonomer/catalyst by said beads is at least equivalent to the rate ofaddition of emulsified comonomer/catalyst solution, then allowing saidsubpension to stand for 30 minutes to 4 hours, and then raising thetemperature to 80°-95° C., and maintaining the suspension at saidtemperature to polymerize said comonomers; and (g) heating saidsuspension to a temperature of 115° to about 135° C. to substantiallycomplete the polymerization of said styrene monomer and divinylbenzeneto form beads having copolymer relatively uniformly distributedthroughout the beads.
 2. A bead product comprising a polystyrene beadhaving a styrene monomer-divinylbenzene copolymer relatively uniformlydispersed throughout the bead; said copolymer consisting essentially ofstyrene monomer and 0.02 to 0.85 percent by weight of divinylbenzene andsaid copolymer comprising 8 to 35 percent by weight of the total beadproduct; said bead product produced by the process comprising:(a)forming an aqueous suspension of initial styrene polymer beads having aweight average molecular weight of 230,000 to 350,000 with apolydispersity (Mw/Mn) of 2 to 3.1, with the aid of a suitablesuspending agent system; said initial styrene polymer beads beingselected from the group condisting of polymers of styrene,alpha-methylstyrene, nuclear-methylstyrene, para-tert-butylstyrene,chlorostyrene, dischlorostyrene, mixtures of these, and copolymers of atleast 50% styrene and up to 50% of other mono-ethylenically unsaturatedmonomers copolymerizable therewith; (b) forming a comonomer solution ofstyrene monomer and divinylbenzene; said styrene monomer being selectedfrom the group consisting of styrene, alpha-methylstyrene,nuclearmethylstyrene, para-tert-butylstyrene, monochlorostyrene,dichlorostyrene and mixtures thereof; (c) forming an emulsion comprisingsuitable emulsifier and a portion of said comonomer solution withfree-radical-producing catalysts dissolved therein; (d) adding theemulsion to the styrene polymer bead suspension at 25°-75° C. andallowing said suspension to stand for 30 minutes to 4 hours to allow thecomonomers and catalysts to be absorbed into the beads; (e) heating thesuspension to 80°-95° C. and maintaining said temperature for 0.10 to 6hours to copolymerize said comonomers; and (f) heating said suspensionto a temperature of 115° to about 135° C. to substantially complete thepolymerization of said styrene monomer and divinylbenzene to form beadshaving copolymers relatively uniformly distributed throughout the beads.